Cavitation in a liquid, with a corresponding release of sound energy and/or light energy, is a common occurrence where a perturbing force is present in the liquid. This force can arise from a locally random perturbation, such as a stirring motion or similar action introduced at selected portions (less than all) of the liquid; or the force can arise from perturbations that are coherent over short distances, such as a liquid perturbation intentionally introduced by a transducer immersed in the liquid. One problem is discrimination of the energy pulses produced by the perturbation of interest from energy pulses introduced by other agencies that are not of interest. Since about 1920, it has been known that one or more electromagnetic pulses are released when a cavitation void or bubble collapses, and that the pulse(s) of energy released varies with the degree of vacuum in the void. In the book Sonoluminescence and Sonochemistry edited by Lawrence A. Crumm et al, Kluwer Academic Publishers, Dordrecht, 1999, several workers discuss some of the problems and interferences that must be dealt with in probing a given liquid for cavitation action.
One particular use of cavitation is in combination with the amplitude and frequency of sonic vibration generated by ultrasonic and megasonic cleaners to remove particles on silicon wafers and other semiconductor surfaces. Cavitation is the rapid formation and explosion of tiny gas bubbles in a liquid due to the pressure waves generated by a vibrating transducer in a cleaner. Particle removal can be accomplished without surface damage when cavitation occurs uniformly across the wafer surface and its density is controlled. However, in many cavitation systems cavitation occurs non-uniformly, often at certain specific sites on the wafer, leading to pitting and damage. There is currently no means for measuring cavitation uniformity or density in the vicinity of specific sites on the wafer in real time.
What is needed is a system that (1) provides an accurate measurement of the number of cavitation events present in a selected volume of a liquid and (2) discriminates against, or substantially eliminates, the effect of cavitation events that occur elsewhere, not within the selected volume. Preferably, the system should be flexible enough to allow location dependent cavitation probing and should provide compensation for cavitation events of interest that occur within the selected volume but are not sensed by the probe apparatus, due to physical or geometrical constraint.